Optical fiber and optical cable

ABSTRACT

The present invention relates to an optical fiber and an optical cable which can be used for a long term even under environments in which an oil content migrates into them, and the optical fiber has a glass fiber extending along a predetermined axis, and a coating. The coating is composed of a plurality of layers each of which is comprised of an ultraviolet curable resin or a thermosetting resin, and swelling rates of the respective coating layers are set so that they increase from an outer peripheral surface of the glass fiber to an outer peripheral surface of the cable jacket.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical fiber and an optical cableincluding a glass fiber.

2. Related Background Art

Studies have been conducted in recent years on uses of optical fiber invery short-range areas as well, e.g., uses in industrial robots andautomobiles, and optical fiber cables obtained by coating an opticalfiber with resin have been used under high-temperature environments inwhich oil or a PVC (polyvinyl chloride) electric cable is present aroundthem, particularly, in the uses in industrial robots and automobiles.

For example, Japanese Patent Application Laid-Open Publication No.2012-223013 (Literature 1) discloses an example of a harness in which anoptical fiber cable and an electric cable are bundled. In view of theforegoing use environments, an optical fiber with superior resistance toethanol is disclosed in Japanese Patent Application Laid-OpenPublication No. 2006-133669 (Literature 2) and an overcoated opticalfiber easy to remove an overcoat layer is disclosed in Japanese PatentApplication Laid-Open Publication No. 2007-199525 (Literature 3). Ajacket of the optical fiber disclosed in Literature 2 is set so that acoating layer located inside has a larger swelling rate than a coatinglayer located outside. The overcoated optical fiber disclosed inLiterature 3 is set so that a coating layer located inside has a smallercrosslink density than a coating layer located outside.

SUMMARY OF THE INVENTION

The Inventors conducted research on the conventional optical cables andfound the problem as described below. Namely, when the optical cable isused under the high-temperature environment in which oil or the PVCelectric cable is present around it, a plasticizer with a low molecularweight migrates into the optical fiber, which caused such a trouble thatthe coating of the optical fiber became cracked in long-termdeterioration evaluation.

FIG. 1 is a drawing for explaining a state of the aforementioned trouble(migration of plasticizers from PVC electric cables into an opticalfiber). FIG. 1 shows a situation (cross-sectional view) in which anoptical cable 1 and PVC electric cables 2A, 2B are set in contact. Theoptical cable 1 has an optical fiber 10 (one in which a glass fiberincluding a core and a cladding is coated with resin, or, one in which ahermetic coat layer is further provided on an outer peripheral surfaceof the cladding and is coated with resin), and a cable jacket 20surrounding the optical fiber 10. The PVC electric cable 2A has a metalsignal wire 20A and a resin layer 21A surrounding it, and the PVCelectric cable 2B has a metal signal wire 20B and a resin layer 21Bsurrounding it. It should be noted herein that the cable jacket 20 isnot always set in contact with the optical fiber 10. The optical cablemay be one in which aramid fiber is set along and around the peripheryof the optical fiber 10 and the cable jacket 20 is laid over it.

As shown in FIG. 1, while the optical cable 1 is placed together withthe PVC electric cables 2A, 2B under the high-temperature environmentfor a long term, the plasticizers in the PVC electric cables 2A, 2B oroil migrates through the optical cable jacket 20 into the optical fiber10. The melting points of the plasticizers are lower than that of thecable jacket 20 and thus the plasticizers are more likely to migrate asthe molecular weights thereof become smaller. It is difficult to preventthe migration, particularly, of the plasticizers having the molecularweight of not more than 1000. A plurality of coating layers constitutingthe cable jacket 20 are comprised of urethane acrylate or epoxy acrylateand the migrating plasticizers come into spaces between molecules ofthese resins. For this reason, as long as crosslink points of moleculesare firm, the cable jacket 20 itself hardly becomes cracked even withthe plasticizers migrating to cause swelling. However, as the crosslinkpoints start breaking because of hydrochloric acid emanating from PVC,the cable jacket 20 becomes easier to crack due to the swelling.

The present invention has been accomplished to solve the above problemand it is an object of the present invention to provide an optical fiberand an optical cable with a structure for enabling long-term use withoutdeterioration of the coating such as occurrence of cracking, even inenvironments in which the plasticizer with a low molecular weightmigrates into the optical fiber side.

In order to solve the above problem, an optical fiber according to anembodiment of the present invention comprises a glass fiber extendingalong a central axis, and a coating surrounding an outer peripheralsurface of the glass fiber. The coating is composed of a plurality oflayers laid along a radial direction from the central axis of theoptical fiber and each of the plurality of coating layers is comprisedof an ultraviolet curable resin or a thermosetting resin. The glassfiber comprises at least a core functioning as a signal transmissionline. A cladding surrounding the outer periphery of the core iscomprised of glass or resin. The optical fiber may further comprise ahermetic coat layer comprised of a low-melting-point glass surroundingan outer peripheral surface of the cladding, in addition to the core andthe cladding.

Particularly, in a first aspect of the present embodiment, two coatinglayers selected from the plurality of layers constituting the coatingare designed as to swelling rates thereof with a plasticizer forpolyvinyl chloride resin so that an inside coating layer closer to theglass fiber has the smaller swelling rate than an outside coating layerfarther from the glass fiber than the inside coating layer. Therefore,in cases where the coating is composed of three or more layers, thelayers are designed as to the swelling rates thereof with theplasticizer for polyvinyl chloride resin or the like so that theswelling rates successively increase from the coating layer in contactwith the outer peripheral surface of the glass fiber to the coatinglayer located outermost.

As a second aspect applicable to the first aspect, when the insidecoating layer and the outside coating layer are adjacent coating layersin contact with each other, the inside coating layer and the outsidecoating layer preferably satisfy the following relation:

(d1/2)×(1+α1)≦(d2/2−t2)×(1+α2),

where, in a cross section of the optical fiber cable perpendicular tothe central axis, d1 represents an outer diameter of the inside coatinglayer, t1 a thickness of the inside coating layer, α1 the swelling rateof the inside coating layer, d2 an outer diameter of the outside coatinglayer, t2 a thickness of the outside coating layer, and α2 the swellingrate of the outside coating layer.

As a third aspect applicable to at least either one of the first andsecond aspects, the plasticizer is preferably a plasticizer forpolyvinyl chloride. As a fourth aspect applicable to at least either oneof the first and second aspects, the plasticizer preferably contains atleast any one of phthalate, dioctyl phthalate (DOP or DEHP), diisononylphthalate (DINP), diisodecyl phthalate (DIDP), dibutyl phthalate (DBP),adipate, dioctyl adipate (DOA or DEHA), diisononyl adipate (DINA),trimellitate, trioctyl trimellitate (TOTM), polyester, phosphate,tricresyl phosphate (TCP), citrate, acetyl tributyl citrate (ATBC),epoxidized oil, epoxidized soybean-oil (ESBO), epoxidized linseed-oil(ELSO), sebacate, and azelate.

In a fifth aspect of the present embodiment, two coating layers selectedfrom the plurality of layers constituting the coating are designed sothat a crosslink density of an inside coating layer closer to the glassfiber is larger than a crosslink density of an outside coating layerfarther from the glass fiber than the inside coating layer. In the fifthaspect as well, in the cases where the coating is composed of three ormore layers, the coating layers are designed as to the crosslinkdensities thereof so that the crosslink densities successively decreasefrom the coating layer in contact with the outer peripheral surface ofthe glass fiber to the coating layer located outermost.

Furthermore, in a sixth aspect of the present embodiment, two coatinglayers selected from the plurality of layers constituting the coatingare designed as to an elongation at break thereof so that an insidecoating layer closer to the glass fiber has the smaller elongation atbreak than an outside coating layer farther from the glass fiber thanthe inside coating layer. In the sixth aspect as well, in the caseswhere the coating is composed of three or more layers, the coatinglayers are designed as to the elongation at break thereof so that theelongation at break successively increase from the coating layer incontact with the outer peripheral surface of the glass fiber to thecoating layer located outermost.

An optical cable according to an embodiment of the present inventioncomprises the optical fiber according to at least any one of the abovefirst to sixth aspects, and a cable jacket of resin provided around thecoating of the optical fiber.

Each of embodiments according to the present invention will become morefully understood from the detailed description given hereinbelow and theaccompanying drawings. These embodiments are presented by way ofillustration only, and thus are not to be considered as limiting thepresent invention.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, and it is apparent that variousmodifications and improvements within the scope of the invention wouldbe obvious to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing for explaining a state of migration of plasticizersfrom PVC electric cables into an optical fiber.

FIG. 2 is a drawing showing an example of a cross-sectional structure ofan optical cable according to an embodiment of the present invention.

FIGS. 3A to 3C are drawings showing examples of various optical fibersapplicable to an optical fiber of the optical cable according to theembodiment shown in FIG. 2.

FIG. 4 is a drawing for explaining structural parameters of the opticalfiber according to the embodiment.

FIG. 5 is a graph for explaining a magnitude relation of swelling ratesbetween coating layers.

FIG. 6 is a graph for explaining a magnitude relation of crosslinkdensities between coating layers.

FIG. 7 is a graph for explaining a magnitude relation of elongation atbreak between coating layers.

DETAILED DESCRIPTION OF EMBODIMENTS

Each of embodiments of the optical fiber and optical cable according tothe present invention will be described below in detail with referenceto the accompanying drawings. The same elements will be denoted by thesame reference signs in the description of the drawings, withoutredundant description.

When an optical cable is used under the aforementioned high-temperatureenvironment in which oil or the PVC electric cable is present around it,an example of cracking occurring in the coating is assumed to be suchthat the oil or the plasticizer (phthalate or the like) for PVC migratesinto resin with a low modulus of elasticity to swell each of a pluralityof layers (resin layers) constituting the coating, resulting in breakageof the coating layer with a low elongation at break. In a configurationwherein the coating layer located outside among the plurality of coatinglayers is more likely to become swollen, the entire coating is unlikelyto crack, whereas in a configuration wherein the coating layer locatedinside among the plurality of coating layers is more likely to becomeswollen, the coating layer located outside is forcibly expanded betweenmolecules because of the swelling of the coating layer located inside,so as to possibly result in cracking of the entire coating. For thisreason, the optical fiber cable with a plurality of coating layers onthe outer peripheral surface of the glass fiber according to the presentinvention is designed as to swelling rates with the plasticizer for PVCso that the swelling rate of the outside coating layer located outsideis set larger than that of the inside coating layer in contact with theglass fiber (in a configuration provided with a core and a cladding orin a configuration further provided with a hermetic coat layer), or, sothat even if the swelling rate with the plasticizer for PVC or the like,of the outside coating layer located outside is smaller than that of theinside coating layer in contact with the glass fiber, an elongation atbreak of the outside coating layer is set larger than that of the insidecoating layer. A difference between the elongation at break of theinside coating layer and the outside coating layer is made dependingupon a level of a difference between the swelling rates of the insidecoating layer and the outside coating layer, whereby, even withelongation of the outside coating layer due to the swelling of theinside coating layer, the elongation of the outside coating layer iskept within the range of the elongation at break thereof. The sameeffect is also achieved by such setting that the crosslink density ofthe inside coating layer is set larger than that of the outside coatinglayer. A difference between the crosslink densities of the insidecoating layer and the outside coating layer is made depending upon alevel of the difference between the swelling rates of the inside coatinglayer and the outside coating layer. The difference between thecrosslink densities of the two layers is made so that the inside coatinglayer and the outside coating layer are swollen without occurrence ofcracking of the outside coating layer, while the swell of the insidecoating layer is more suppressed by the degree of the difference of thecrosslink density of the inside coating layer from the other, than theswell of the outside coating layer.

A specific structure of the optical cable according to an embodiment ofthe present invention will be described below. FIG. 2 is a drawingshowing an example of a cross-sectional structure of an optical cableaccording to an embodiment of the present invention. FIGS. 3A to 3C aredrawings showing examples of optical fibers having variouscross-sectional structures, which are applicable to an optical fiber 100in the optical cable 200 in FIG. 2. For example, FIG. 3A is a drawingshowing an example of a cross-sectional structure of an optical fiber100A according to an embodiment of the present invention, FIG. 3B adrawing showing an example of a cross-sectional structure of an opticalfiber 100B according to an embodiment of the present invention, and FIG.3C a drawing showing an example of a cross-sectional structure of anoptical fiber 100C according to an embodiment of the present invention.

As shown in FIG. 2, the optical cable 200 of the present embodiment hasthe optical fiber 100 extending along the central axis (optical axis AX)(which is the optical fiber according to the present embodiment), and acable jacket 210 of resin as a coating on the optical fiber 100. Theoptical fiber 100 has a glass fiber 110, and a coating 150 surroundingthe glass fiber 110 and consisting of a plurality of resin layers. Avariety of cross-sectional structures as shown in FIGS. 3A to 3C areapplicable to the optical fiber 100.

The optical cable 100A shown in FIG. 3A is composed of a glass fiber110A extending along the central axis (optical axis AX), and the coating150 surrounding the glass fiber 110A. In the optical fiber 100A, theglass fiber 110A has a core 111 functioning as a light transmission lineextending along the central axis, and a cladding 112 surrounding thecore 111. The coating 150 has an inside coating layer 120 in contactwith the glass fiber 110A, and an outside coating layer 130 providedoutside the inside coating layer 120.

The cladding 112 can be made of a plastic material such as urethane(meth)acrylate resin and in this case, the glass fiber is composed ofonly the core, as shown in FIG. 3B. The optical fiber 100B shown in FIG.3B has a glass fiber 110B composed of only the core 111, the cladding112 surrounding the glass fiber 110B and comprised of the plasticmaterial, and the coating 150 surrounding the cladding 112. Thestructure of the coating 150 in this optical fiber 100B may be composedof one layer, may be the same as the coating 150 of the optical fiber100A shown in FIG. 3A, or may be composed of three or more coatinglayers.

On the other hand, the optical fiber 100C shown in FIG. 3C is differentin the structure of a glass fiber 110C from the optical fiber 100A.Namely, the optical fiber 100C has the core 111 functioning as a lighttransmission line extending along the central axis (optical axis AX),the cladding 112 surrounding the core 111, and a hermetic coat layer 113surrounding the cladding 112. The coating 150 of the optical fiber 100Calso has the inside coating layer 120 in contact with the hermetic coatlayer 113, and the outside coating layer 130 provided outside the insidecoating layer 120 as in the structure shown in FIG. 3A. It is a matterof course that the coating 150 in FIG. 3C may be composed of three ormore coating layers.

FIG. 4 shows a cross-sectional front view of the optical fibers 100A to100C (corresponding to the optical fiber 100 of the optical cable 200)according to the present embodiment. Namely, the coating layer 120(inside coating) provided on the outer periphery of the glass fiber 110Aor 110C (or the plastic cladding 112 surrounding the glass fiber 110B)has the outer diameter d1 and the thickness t1 along the radialdirection. The coating layer 130 (outside coating) provided outside thecoating layer 120 has the outer diameter d2 and the thickness t2 alongthe radial direction. As the coating layers 120, 130 become swollen,stresses of both of inward expansion and outward expansion act in theradial direction from the optical axis AX. In each coating layer, theinward expansion in the coating layer is released to the outward by theswell and the inward also expands as pulled by the force. However, whenthe swelling rate of the coating layer 120 located inside is higher thanthat of the coating layer 130 located outside, the coating layers 120,130 both are subject to compressive stress. The entire coating is likelyto expand outward from a steady state, in order to release thecompressive stress. If in this state crosslink points in each coatinglayer are broken because of hydrochloric acid, ultraviolet light, heat,and so forth, the coating can crack or break eventually.

Then, the present embodiment involves setting the swelling rates of thecoating layers 120, 130 in contact with each other as shown in FIG. 4,among a plurality of layers constituting the coating (which may be threeor more layers), so as to satisfy the following relation:

(d1/2)×(1+α1)≦(d2/2−t2)×(1+α2),

thereby to prevent the breakage of the coating in the optical fibers100A to 100C.

A swelling rate is measured by immersing an optical fiber as an objectin a plasticizer (liquid) and measuring volumes or weights before andafter the immersion, and is expressed as a percentage of a ratio of thevolumes or weights before and after the immersion. The plasticizer to beused may be a plasticizer for PVC or the like (e.g., phthalate).

FIG. 5 is a graph for explaining the magnitude relation of swellingrates between the coating layers 120, 130. Particularly, the presentembodiment is designed as to the swelling rates with the plasticizer forpolyvinyl chloride resin so that the swelling rate of the coating layer120 (inside coating) is smaller than that of the coating layer 130(outside coating). In cases where the coating is composed of three ormore layers, the swelling rates thereof with the plasticizer forpolyvinyl chloride resin are designed, as indicated by a dashed line inFIG. 5, so that they successively increase from the coating layer incontact with the outer peripheral surface of the glass fiber 110A or110C (or the plastic cladding 112 surrounding the glass fiber 110B) tothe coating layer located outermost in the cable jacket. Phthalate isknown as typical plasticizer for polyvinyl chloride resin.

FIG. 6 is a graph for explaining the magnitude relation of crosslinkdensities between the coating layers 120, 130. The present embodiment isdesigned so that the crosslink density of the coating layer 120 (insidecoating) is larger than that of the coating layer 130 (outside coating).In the cases where the coating 150 is composed of three or more layers,the crosslink densities thereof are designed, as indicated by a dashedline in FIG. 5, so that they successively decrease from the coatinglayer in contact with the outer peripheral surface of the glass fiber110A or 110C (or the plastic cladding 112 surrounding the glass fiber110B) to the coating layer located outermost in the cable jacket.

A crosslink density of a cured product (each coating layer in the caseof the present embodiment) is obtained from the following equation bymeasurement of dynamic viscoelasticity.

ρ=G′/φRT

In this equation, G′ represents the storage elastic modulus attemperature T, φ the front factor (assumed to be 1), R the gas constant,and T a temperature which is not less than Tg and at which G′ becomesapproximately flat against temperature, when measured by DMA (DynamicMechanical Analysis), the temperature T being given approximately asT=Tg (glass transition temperature)+30° C. The glass transitiontemperature Tg is measured by Differential Scanning Calorymetry (DSC)and Thermo Mechanical Analysis (TMA).

Furthermore, FIG. 7 is a graph for explaining the magnitude relation ofelongation at break between the coating layers 120, 130. The presentembodiment is designed as to the elongation at break with theplasticizer for polyvinyl chloride resin so that the elongation at breakof the coating layer 120 (inside coating) is smaller than that of thecoating layer 130 (outside coating). In the example of FIG. 7 as well,in the cases where the coating is composed of three or more layers, theelongation at break thereof with the plasticizer for polyvinyl chlorideresin are designed so that they successively increase from the coatinglayer in contact with the outer peripheral surface of the glass fiber110A or 110C (or the plastic cladding 112 surrounding the glass fiber110B) to the coating layer located outermost.

The measurement of each elongation at break with the plasticizer iscarried out by a method conforming to AMST D882 Standard, to measure theelongation break with the plasticizer while a film is kept in contactwith the plasticizer.

(Plasticizer)

The plasticizer applicable to PVC will be described below. Plasticizeris the general term of additive chemicals to be added in thermoplasticsynthetic resin to improve flexibility and weatherability thereof, andit is also used for making PVC more flexible.

In general, a thermoplastic resin has a glass transition temperature(also called a glass transition point), and the resin exhibitswell-ordered crystallinity of molecular arrangement at temperaturesbelow the glass transition temperature but an amorphous state ofmolecular arrangement in a temperature zone from the glass transitiontemperature to a melting point. The thermoplastic resin in the amorphousstate demonstrates flexibility and high optical transparency and thus isuseful in many applications. On the other hand, the crystalline resin isopaque and becomes fragile against impact and external force withprogress of crystallinity at low temperatures, often demonstratingproperties deemed as disadvantages in use of the resin.

Since the melting point and glass transition point are determined by atype of the resin and the degree of polymerization thereof, thetemperature characteristics of the resulting resin do not always agreewith those of a desired product. Addition of an additive in thethermoplastic resin expands the temperature zone of the amorphous stateto prevent fragility from appearing even at low temperatures and toenhance flexibility, realizing the resin with desired temperature andphysical properties. A chemical added for this purpose is theplasticizer. The plasticizer further increases elasticity, thereby toimprove moldability as well; for example, it becomes easier to release amolded product from a die during injection molding.

The plasticizer comes into spaces of the resin to inhibit the resin frombeing regularly oriented, whereby the resin is maintained in theamorphous state even at temperatures below the glass transition point.Therefore, the plasticizer having large side chains often demonstratesuseful properties. If the plasticizer is incompatible with the objectiveresin, phase separation will occur between the resin and theplasticizer; therefore, the plasticizer needs to have a characteristicof wide compatibility with various resins while causing no phaseseparation.

Particularly, in the case of polyvinyl chloride (PVC), products with awide variety of properties are prepared by addition of the plasticizer.Typical examples of the plasticizer used include phthalates, among whichDEHP and DINP have properties as ideal general-purpose plasticizers andare manufactured in large quantities.

(Examples of Plasticizer)

Examples of the plasticizer include phthalate, dioctyl phthalate (DOP orDEHP), diisononyl phthalate (DINP), diisodecyl phthalate (DIDP), dibutylphthalate (DBP), adipate, dioctyl adipate (DOA or DEHA), diisononyladipate (DINA), trimellitate, trioctyl trimellitate (TOTM), polyester,phosphate, tricresyl phosphate (TCP), citrate, acetyl tributyl citrate(ATBC), epoxidized oil, epoxidized soybean-oil (ESBO), epoxidizedlinseed-oil (ELSO), sebacate, and azelate.

Phthalate is the general term of esters of phthalic acid (ortho-isomer)and alcohol. Phthalates of higher alcohols typified bybis(2-ethylhexyl)phthalate are useful as plasticizers (phthalicacid-based plasticizers). Industrially in general, phthalic acid isesterified by azeotropic dehydration of water and alcohol from phthalicacid (free acid) and excess alcohol. Table 1 below provides theabbreviation, molecular weight, melting point (° C.), boiling point (°C.), and CAS number of each of major compounds of phthalates.

TABLE 1 Melting Boiling point point Compound name Abbr. Mol. wt. (° C.)(° C.) CAS number Dimethyl phthalate DMP 194.19 2 282 [131-11-3] Diethylphthalate DEP 222.24 −3 289-299 [84-66-2] Diallyl phthalate DAP 246.26−70 165-167(*) [131-17-9] Dibutyl phthalate DBP 278.35 −35 340 [84-74-2]Diisobutyl phthalate DIBP — 327 [84-69-5] Di-n-hexyl phthalate DHP — —[84-75-3] Dioctyl phthalate DOP 390.56 −50 384 [117-81-7]Bis(2-ethylhexyl)phthalate DEHP Di-n-octyl phthalate DnOP 390.56Diisononyl phthalate DINP 418 403 [28553-12-0] [68515-48-0] Dinonylphthalate DNP 418 Diisodecyl phthalate DIDP 446 −50 420 [26761-40-0]Benzyl butyl phthalate BBP 312 370 Bis(butylbenzyl)phthalate BBzP (*)°C./5 mmHg

(Materials of Coating Layers)

Constituent materials of the coating 150 in the optical fiber of thepresent embodiment will be described below. Each of the coating layers120, 130 is comprised of an ultraviolet (UV) curable resin or athermosetting resin and types and properties of the UV curable resinsapplicable to each coating layer will be described below.

The UV curable resins are roughly classified into the radicalpolymerization type of acrylate and the cation polymerization type ofepoxy. The radical polymerization type consists primarily of acrylateand has a cure shrinkage rate of 5 to 10%. The radical polymerizationtype is subject to curing inhibition by oxygen, curing reaction alsostops after termination of irradiation with UV, and the curing is lessaccelerated by heat. Furthermore, the radical polymerization type ischaracterized by moderate thermal resistance and moderate chemicalresistance, and has a large degree of freedom of resin design. On theother hand, the cation polymerization type consists primarily of epoxyand has a cure shrinkage rate of 2 to 4%. The cation polymerization typeis free of the curing inhibition by oxygen, the curing reactioncontinues even after termination of irradiation with UV, and the curingis accelerated by heat. Furthermore, the cation polymerization type ischaracterized by good thermal resistance and good chemical resistancebut has a small degree of freedom of resin design.

The radical polymerization type is further classified under epoxyacrylate, urethane acrylate, and silicone acrylate.

The cable jacket 210 can be comprised of a thermoplastic resin, e.g., apolyolefin-based resin such as polyethylene or polypropylene, orpolyamide. The cable jacket 210 imparts mechanical strength to theoptical fiber 100 (100A-100C). The diameter of the optical fiber 100(100A-100C) can be in the range of 0.25 to 0.5 mm, and the diameter ofthe optical cable 200 in the range of 1 to 3 mm. The thickness of thecable jacket 210 can be in the range of 0.3 to 1 mm. Aramid fiberfunctions as a tension member of the optical cable 200 and Kevlar(registered trademark) or the like is available.

As constructed as described above, the optical fiber and the opticalcable according to the present invention can be used for a long termwithout deterioration of the coating such as cracking of the cablejacket, even in the environments in which the oil content such as theplasticizer with a low molecular weight migrates into the optical fiberside.

From the above description of the present invention, it will be obviousthat the invention may be varied in many ways. Such variations are notto be regarded as a departure from the spirit and scope of the presentinvention, and all improvements as would be obvious to those skilled inthe art are intended for inclusion within the scope of the followingclaims.

1. An optical fiber comprising: a glass fiber; and a coating surroundingthe glass fiber, wherein the coating is laid on the glass fiber along aradial direction from a central axis of the optical fiber and comprisesan inside coating layer and an outside coating layer surrounding theinside coating layer, wherein the inside coating layer is comprised ofan ultraviolet curable resin or thermosetting resin which has a swellingrate with a plasticizer for polyvinyl chloride resin, wherein theoutside coating layer is comprised of an ultraviolet curable resin orthermosetting resin which has a swelling rate with a plasticizer forpolyvinyl chloride resin, wherein the swelling rate of the insidecoating layer is smaller than the swelling rate of the outside coatinglayer, and wherein the plasticizer contains at least any one ofphthalate, dioctyl phthalate, diisononyl phthalate, diisodecylphthalate, dibutyl phthalate, adipate, dioctyl adipate, diisononyladipate, trimellitate, trioctyl trimellitate, phosphate, tricresylphosphate, citrate, acetyl tributyl citrate, epoxidized oil, epoxidizedsoybean-oil, epoxidized linseed-oil, sebacate, and azelate.
 2. Theoptical fiber according to claim 1, wherein the inside coating layer andthe outside coating layer are adjacent coating layers in contact witheach other. 3-6. (canceled)
 7. An optical cable comprising: the opticalfiber as defined in claim 1; and a cable jacket of resin provided aroundthe coating of the optical fiber. 8-9. (canceled)
 10. The optical fiberaccording to claim 1, a crosslink density of the inside coating layer islarger than a crosslink density of the outside coating layer.
 11. Anoptical cable comprising: the optical fiber as defined in claim 10; anda cable jacket of resin provided around the coating of the opticalfiber.
 12. The optical fiber according to claim 1, wherein, anelongation break of the inside coating layer is smaller than anelongation at break of the outside coating layer.
 13. An optical cablecomprising: the optical fiber as defined in claim 12; and a cable jacketof resin provided around the coating of the optical fiber.